There are many cases in which a metal (for example, steel) is coated and used for improving durability or for obtaining a beautiful external appearance, and a coated metal is widely used in various fields such as home electronics, automobiles, building materials, and outdoor structures. It is necessary that the metal has superior contamination resistance in addition to corrosion resistance, because the metal is exposed to rain, wind, dust, and the like, particularly when being used in outdoors.
Photocatalytic technique is a technique of dispersing particles having superior photocatalytic activity in a coating film of a material surface in order to decompose and remove contaminants mainly composed of organic materials. In this technique, the particles having superior photocatalytic activity have a high effect on the decomposition of contaminants of the surface. But also this technique deteriorates a coating film by gradually decomposing the resin-based coating film which is an organic material. Therefore, it is difficult to use the coating film for a long period of time without any change. Accordingly, in order to minimize deterioration of the coating film, various proposals have been made.
For example, a method of using an inorganic material as a matrix is disclosed in Patent Documents 1 and 2. In addition, since a fluororesin among organic materials for coating films is relatively stable to a photocatalyst, a method of using a fluororesin as a matrix is disclosed (Patent Document 3). In addition, in order to obtain high stability to a photocatalyst and workability which are required particularly for a precoated metal, a technique of using a silica-organosilane material as a matrix is disclosed in Patent Documents 4 and 5 as a method of using an acrylic silicate, which is obtained by a polymerization reaction of an acrylic resin and an organoalkoxysilane, as a matrix. In addition, a method using a vinylidene fluoride resin and an acrylic resin is disclosed in Patent Document 6.
Meanwhile, since particles (hereinafter, referred as “photocatalytic particles”) having photocatalytic activity are almost uniformly dispersed in a film, the concentration of the photocatalytic particles in a surface of the film is not necessarily high. In addition, while the photocatalytic particles are dispersed in the film, surfaces of the particles are covered with a matrix resin. Therefore, even when the photocatalytic particles are present near the surface of the film, contaminants on the surface may not be decomposed immediately after usage starts.
In addition, regarding the photocatalytic film, while the film is used for a certain period under sunlight or ultraviolet light, a resin near a surface of the film is slightly decomposed and impaired by a photocatalytic effect and thus, surfaces of photocatalytic particles are exposed to the outside, thereby superior contamination resistance is exhibited. However, as described above, when a stable resin to a photocatalyst is used as a matrix resin, it is difficult to advance the decomposition and deterioration of the resin. Therefore, a long period of time is required in order to exhibit superior contamination resistance. Accordingly, in a coated metal, it is difficult to exhibit superior contamination resistance from the initial stage immediately after usage starts, and to suppress the decomposition and deterioration of a matrix resin of a film and thus maintain superior contamination resistance for a long period of time.